Dr van Veen - Chemotherapy Flashcards
0
Q
What are narrow spectrum antibiotics?
A
Those which are mainly effective against either Gram-negative or Gram-positive bacteria
1
Q
What are broad spectrum antibiotics?
A
Those which target a wide range of Gram-positive and Gram-negative bacteria
2
Q
What are limited spectrum antibiotics?
A
Those which are effective against a single organism or disease
3
Q
What are the features of later generations of semisynthetic variants of antibiotics
A
Increased oral bioavailabililty
Increased stability
broader spectrum (extended spectrum)
Efficacy against resistant microorganisms
4
Q
What are the five main classes of antibiotics?
A
Cephalosporins Macrolides β-lactamase inhibitors Penicillins Quinolones
5
Q
What are the two general categories of antibiotic action?
A
Bacteriostatic vs bactericidal
6
Q
Give an example of a bacteriostatic and a bactericidal antibiotic
A
- cidal = penicillin
- static = chloramphenicol
7
Q
What are two measures of effectiveness of a chemotherapeutic drug?
A
Minimal inhibitory concentration
Minimal bactericidal concentration
8
Q
What are the major targets of anitbiotic action?
A
Cell wall biosynthesis Protein biosynthesis DNA replication, repair and expression Folate coenzyme biosynthesis (Also in some cases membranes can be the target)
9
Q
Explain the biosynthesis of peptidoglycans
A
- UTP and three amino acids joined to N-acetylglucosamine => UDP N-acetyl muramyl-tripeptide
- D-Ala-D-Ala is joined to produce UDP N-acetyl muramyl pentapeptide
- UDP N-a m pp is then joined to UDP N-acetylglucosamine (also linked to bactoprenol phosphate via a phosphate bridge = association with the inner leaflet of the phospholipid bilayer and transports it to the outer leaflet)
- PG repeat units are joined to make a polymer and bactoprenol carrier is detached
- PG polymers cross linked by peptidoglycan transpeptidase
- Reorientation of bactoprenol pyrophosphate to the inner membrane surface and its dephosphorylation to bactoprenol phosphate
10
Q
D-cycloserin
A
Antibiotic that is a structural analogue of D-alanine
Prevents formation of the pentapeptide through inhibition of
- L-alanine racemase
- D-alanyl-D-alanine synthetase
- Ligase that connects the D-alanyl-D-alanine unit to the muramyl-tripeptide (possibly)
11
Q
Fosfomycin
A
Antibiotic that inhibits cell wall biosynthesis through inhibition of pyruvyl transferase
(transfers the phosphoenolpyruvate group to UDP N-acetylglucosamine in the production of UDP N-acetyl muramyl-tripeptide)
12
Q
Penicillin
A
β-lactam antibiotic
Inhibits the PG-cross-linking transpeptidases
13
Q
Ampicillin
A
β-lactam antibiotic
Inhibits the PG-cross-linking transpeptidase
14
Q
What kind of enzyme are transpeptidases? How do they work in cell wall biosynthesis?
A
All variants of ‘serine’ hydrolases
- Attack of the active site serine on the amide bond between the two D-Alas
- Acyl transfer to amino moiety of diaminopimelic acid of L-lysine-(glycine) in a neighbouring pentapeptide = cross-linking
-The adduct then collapse to an acyl-O-ser enzyme with release of D-Ala
15
Q
How do β-lactam function?
A
They inhibt transpeptidase enzymes causing them to commit suicide when they start the catalytic cycle with β-lactam antibiotics
Binds to the active site serine - cannot be hydrolysed as water is excluded from the active site
16
Q
Vancomycin
A
Binds to pentapeptide tails in the PG repeating unit terminating in D-Ala-D-Ala = transpeptidase enzyme cannot access it
17
Q
What is the structure of the bacterial ribosome?
A
Two-subunit nucleoprotein (30S and 50S)
2/3 RNA and 1/3 protein
18
Q
Explain initiation of bacterial protein synthesis
A
Formation of the initiation complex - mRNA becomes attached to the 30S subunit (requires intiation factor 3)
formylmethione-charged tRNA then combines with the mRNA-30S ribosomal complex (require initiation factors 1 and 2, and GTP)
50S binds bound GTP is hydrolysed and initiation factors are released
19
Q
Explain the continuation of bacterial protein synthesis
A
Accomplished by repetition of three reactions
50S joins aminoacids on the tRNA through its peptidyltransferase activity and the peptide is attached to the second tRNA
First tRNA moves to exit site (from P site) and is released
tRNA with dipeptide moves from A to P site
20
Q
Tetracycline
A
Broad spectrum antibiotic - used as first line treatment against Mycoplasma sp. and Vibrio cholera
Reversibly binds to the 30S ribosome and inhibits the entry of aminoacyl-tRNA into the acceptor site with the help of bound Mg2+
21
Q
Where is the binding site of tetracycline? How does this binding occur?
A
30S subunit in a 20A wide and 7A deep groove containing rRNA
The oxygens of the internucleotide phosphodiester links in 16S rRNA form electrostatic interactions, directed through Mg2+
22
Q
What is the structure of aminoglycosides? Upon which bacteria are they effective?
A
Consist of 2 or more sugars linked to an aminocyclitol ring by glycosidic bonds
Narrow spectrum; active uptake in aerobic Gram-negative rods (also act upon some G+ bacteria)
23
Q
What is special about the aminoglycoside antibiotics? Which antibioitics does this inclue
A
They are bactericidal (most protein synthesis inhibitors are purely static) because they insert the ‘wrong’ amino acid in proteins
Streptomycin and gentamycin
24
Streptomycin
Aminoglycoside bactericidal antibiotic (narrow spectrum)
Binds to the 30S ribosomal subunit and freezes the pre-initiation complex Also slows down protein synthesis and can induce misreading
Effective against G- rods (actively taken up)
25
Upon what do macrolides act?
The 50S subunit of the bacterial ribosome
26
Erythromycin
Antibiotic
Inhibits the 50S subunit of the bacterial ribosome by binding to the entrance of the polypeptide exit tunnel
Allows 6-8 oligopeptidyl-tRNA build up before elongation is blocked
27
Chloramphenicol
Broad spectrum antibiotic
Binds to the 50S subunit and inhibits aminoacyl-tRNA binding at the P site
Emergency drug - Sever toxicity
Used in the treatment of bacterial meningitis (haemophilus influenza or Neisseria meningitidis)
28
Fusidic acid
Narrow spectrum antibiotic
Inhibits elongation factor G
Fewer toxic effects than other protein synthesis inhibitors
Narrow spectrum skin and eye infections by G+ bacteria
29
Puromycin
Antibiotic that resembles the 3' end of tyrosyl-tRNA
Enters the A site no the ribosome and transfers to the growing polypeptide chain at the P site causing premature chain termination
Used experimentally
30
What are the three ways in which antibiotics can act on DNA replication and gene expression?
Inhibitors of type II topoisomerases
Inhibitors of RNA synthesis
Compounds that interact with dsDNA
31
What are the inhibitors of type II DNA topoisomerases?
Aminococumarins
Quinoiones
32
Fluoroqiunolones
Synthetic broad spectrum bactericidal antibiotics
Topoisomerase inhibitors
Used in the treatment of Pseudomonas infections (eg cystic fibrosis patients)
33
Ciprofloxacin
Synthetic broad spectrum bactericidal antibiotics
Topoisomerase inhibitors
Used against Bacillus anthrax infections
34
Levofloxacin
3rd gen Fluoroquinolone antibiotic
Broad spectrum and bactericidal
Improved activity against Strep pn.
Really expensive
35
Rifampin
Antibiotic
Binds to β subunit of DNA dep RNA pol of bacteria
Blocks elongation of RNA chain at the stage of initiation by binding to RNA transport tunnel
Used in Myco tuberculosis infections
36
Bleomycin
Antibiotic
Metal-chelating glycopeptide (produces oxygen radicals)
37
Daunomycin
Planar antibiotic molecule that intercalates between nucleobases of duplex DNA
Prevents normal transcription of DNA
38
Mitomycin C
Aziridine antibiotic that acts upon dsDNA
Alkylating agent induces cross-linking between Gs at trans positions in dsDNA
Also shown to act as an anticancer drug (activated through reduction quinone group)
- Alkylation and crosslinking of DNA
39
Streptomycin
Antibiotic
Binds to the 30S subunit
Causes insertion of the wrong amino acid into synthesised proteins
40
Where does tetracycline bind?
30S subunit in rRNA in a groove of 20A wide and 7A deep
41
Lincomycin
Antibiotic
Direct peptidyltransferase inhibitor of bacterial ribosomes
42
What is the target of quinolones?
DNA gyrase (type II topoisomerase)
Also DNA topoisomerase IV
43
Gentamicin
Antibiotic
Binds to the 30S subunit
Causes insertion of the wrong amino acid into synthesised proteins
44
Sulfamethoxazole
Antibiotic
Inhibits dihydropteroate synthase (folic acid synthesis)
45
Valinomycin
Antibiotic
Contains three repeating units of (L-lactate)-(L-valine)-(D-hydroxyisovalerate)-(D-valine) which forms a circle
Carries K+ across the membrane
46
Trimethoprim
Antibiotic
Inhibits dihydrofolate reductase (folic acid synthesis)
47
Gramicidin A
Antibiotic
Hydrophobic linear polypeptide antibiotic with 15 aa and a carboyterminal ethanolamine
Dimerises in the membrane to form an ion channel
48
What is the target of polyenes?
Cell membranes - binding to ergosterol and allowing pore formation
49
Polymixin
Antibiotic - cyclic amphipatic protein with a net charge of 5+
Associates with negatively charged phosphate head groups on the outer surface of the membrane, and then, to aggregate into micelle-like complexes which bind lipids and affect permeability
50
Amphotericin B
Antibiotic
Binds to ergosterol and facilitates formation of pores for ions and macromolecules
51
What is the most important distinction of pharmacological significance to the treatment of parasitic infections? Why is this important?
Whether the infecting organism is unicellular protozoan or multicellular helminth
52
Melarsen
Organic arsenical used against trypanosomes (actively accumulated)
Inhibits lipoic acid-dependent enzymes
Inhibits dithiol containing metallo-enzymes
Affects ATP synthesis
53
Suramin
Development of Ehrlich's Trypan Red
First useful anti-trypanosomal drug without a toxic metal atom
Only effective in early stages as drug does not cross BBB
Mechanism unknown (may be based on inhibition of glycolytic enzymes)
54
What are the drugs used against Leishmaniasis?
Amphotericin B affects membrane permeability
Miconazole inhibits ergosterol biosynthesis
(Leishmania sp's membranes contain ergosterol
55
What do Plasmodium use as a nitrogen source? Why is this important?
Haemoglobin
| Would accumulate to toxic levels if it wasn't polymerised into non-toxic hemozoin
target of chloroquine
56
Chloroqiune
Antimalarial drug
Inhibits the formation of haemozoin (product of their digestion of haemoglobin) resulting in toxic levels of haem (produces ROS)
57
Mefloquine
Antimalarial drug
Inhibits the formation of haemozoin (product of their digestion of haemoglobin) resulting in toxic levels of haem (produces ROS)
58
Artemisinin
Antimalarial agent
Inhibits development of oocysts in mosquitos
Generates highly reactive organic free radicals using peroxide bridge (might inhibit parasite ETC (Cyt b) or SERCA pump)
59
What is a potential target for the treatment of malaria?
Drugs could be used to block the link between the exo-erythrocytic stage thus preventing infection
60
Draw the malarial life cycle and label the potential sites of drug action
Sporogenic cycle
Exo-erythrocytic cycle
Erythrocytic cycle
Act on liver schizont (tafenoquine and primaquine)
Act on oocyst
Act on growth in erythrocytes
Chemoprophylaxis
61
Sulfadoxine
Antimalarial agent
p-aminobenzoate analogue used to inhibit the action of dihydropteroate synthase
62
Pyrimethamine
Antimalarial agent
Trimethoprim analogue that inhibits dihyrofolate reductase
63
Fansidar
Antimalarial agent containing sulfadoxin and pyrimethamine used against P. falciparum
Inhibits both DHFR and DHPS
64
Proguanil
Prophylactic antimalarial prodrug
Converted to cycloguanil which inhibits plasmodial dihydrofolate reductase
65
What two classes of antibiotic have shown activity against certain protozoa?
Tetracylcines and lincomycins
66
What are the targets of antiviral chemotherapies?
Agents that inactivate intact viruses (virucidal)
Agents that modify the host's response to infection (immunomodulating)
Agents that inhibit viral replication at the cellular level (antiviral)
67
Amantadine
Antiviral drug used in the treatment of influenza
Early stages - Blocks the function of the M2 channel protein
Later stages - Interferes with haemagglutinin processing (prevents conformational change)
68
Oseltamivir
Ethyl ester pro-drug used in the treatment of influenza (activated by esterases in the plasma)
Inhibits the neuraminidase of Influenza A and B
Enhances viral aggregation and inhibits release from host cells
Also reduces movement of virus particles through the upper respiratory tract
69
Zanamivir
Drug used in the treatment of influenza
Inhibits the neuraminidase of Influenza A and B
Enhances viral aggregation and inhibits release from host cells
Also reduces movement of virus particles through the upper respiratory tract
70
Draw a cycle of influenza infection and the site at which drugs can inhibits this
Well done
71
Upon what do all available antiherpesvirus agents work
The virally encoded DNA polymerases that replicate the dsDNA genome of the virus
72
Aciclovir
Antiherpes drug that inhibits its DNA polymerase
Monophosphorylated by thymidine kinase and then acts as a chain terminator = permanent inactivation of enzyme
73
How is aciclovir selective for virally infected cells?
Only virally infected cells have the thymidine kinase needed to monophosphorylate the drug
The drug preferentially binds to the virally encoded DNA pol (30x higher affinity)
74
Ganiciclovir
Drug used in the treatment of CMV (phosphorylated by phosphotransferase encoded by CMV)
Competitive inhibitor of DNA polymerase (but has 3' OH moieties)
75
Cidofovir
Nucleoside phosphate analogue of cytosine
Converted to a diphosphoryl derivative that selectively inhibits the DNA polymerase of CMV
76
What are the pyrimidine analogues used in the treatment of herpes virus? How do they function
Trifluridine and idoxuridine
Both inhibit DNA polymerase (but greater toxicity so applied topically)
77
Foscarnet
Organic analogue of pyrophosphate
Selectively binds to viral DNA polymerase of CMV and HSV and prevents the cleavage of PPi from nucleoside triphosphate during DNA pol
78
What are the three kinds of anti HIV agents?
Inhibitors of viral fusion
Reverse transcriptase inhibitors
Protease inhibitors
79
Nevirapine
Anti HIV agent
Binds to reverse transcriptase near catalytic site and denatures it
80
Zidovudine
Anti HIV agent
Binds to reverse transcriptase and acts as a chain terminator (lack of a 3'OH)
Phosphorylated to active form
81
Azidothymidine (AZT)
Anti HIV agent
Binds to reverse transcriptase and acts as a chain terminator (lack of a 3'OH)
Phosphorylated before becoming active
Valyl ester increases bioavailability
82
Why is hydroxyurea often co-administered with pyrimidine analogues?
Inhibits ribonucleotide reductase thereby decreasing the intracellular pool of pyrimidine nucleotides and potentiating the effects of pyrimidine analogues
83
How do HIV protease inhibitors function?
Aspartyl protease converts polypeptides into functioning proteins by cleavage at appropriate positions
Inhibition means that newly produced viruses are not infectious as protease is required during virus budding
84
Saquinavir
HIV protease inhibitor
Binds to the two Asp residues of the protease
Prevents the formation of infectious virus particles
85
Interferon-α-2a
Used in the treatment of hep B and AIDS related Kaposi sarcomas
Low bioavailability so administered intralesionally, SC and IV
Acute flu-like symptoms
86
What are the ways in which anticancer drugs can act?
By preventing effective DNA replication through direct binding to nucleobases or impairing the DNA synthesis machinery
By damaging the mechanisms of cell division such as formation of the mitotic spindles
By blocking the pathways involved in cell growth that are activated by signals such as growth factors of hormones
87
What are the side effects of targeting rapidly proliferating cells?
```
Hair loss
Depression of gametogenesis
Bone marrow suppression
Nausea and vomiting
GI disturbance
```
88
How do nitrogen mustards act as anticancer agents?
Crosslinks are formed between or within DNA strands
Second reaction can occur with water resulting in GC to AT transition
Can also occur with other nucleophilic groups (RNA and protein) = general toxicity
89
Cyclophosphamide
Nitrogen mustard anti cancer drug
Causes cross linking of DNA strands through the generation of a carbonium ion
Taken orally
Metabolised by the liver by P450 to become activated to a phosphoramide mustard
90
Melphalan
Anti cancer drug used against melanomas
Nitrogen mustard attached to phenylalanine = taken up by melanomas (phenylalanine is precursor to melanin
Taken orally
91
Lomustine
Nitrosourea compound used as an anticancer drug
Proposed to act through alkylation of proteins and DNA following generation of an isocyanate molecule and a 2-Chloroethyl diazene hydroxide
92
How do aziridines act as anticancer drugs?
DNA alkylation and cross linking
Activated by chemical or enzymatic reduction of the quinone group
93
Cis-diamine dichloroplatinum (cisplatin)
Anti cancer drugs
Cross linking agent in DNA (site of action are N7 atoms of guanine and adenine)
Small size therefore can only cross link within strands (2Gs)
Only given IV
Has renal toxicity
94
How do anticancer drugs that damage DNA cause cell death?
Through the activation of p53
95
Doxorubicin
Antracyclin anticancer drug
Intercalates with DNA
Also induces oxidative damage to DNA which results in DNA fragmentation
Severe side effects in cardiac tissue due to lipid peroxidation
96
Daunomycin
Antracyclin anticancer drug
Intercalates with DNA
Also induces oxidative damage to DNA which results in DNA fragmentation
Severe side effects in cardiac tissue due to lipid peroxidation
97
Mitoxantrone
Antracyclin anticancer drug
Intercalates with DNA and inhibits topoisomerase type II
Does not result in free radical generation
Used in breast cancer, acute myeloid leukemia and certain lymphomas
98
Methotrexate
Antimetabolite used as an anticancer drug
Inhibits the enzyme dihydrofolate reductase = prevents folate synthesis
Also competes with folic acid for active transport into mammalian cells
Given with leuvocovorin in order to salvage normal tissue from folate depletion (= reduced toxicity
99
5-fluorouracil
Converted in cells into 5-flurouracil-2'-deoxyuridine
Then can inhibit thymidine synthetase
Often given with leucovorin (potentiates action)
100
Leuvocovorin
Given with methotrexate in order to salvage normal tissue from folate depletion
Also given with 5-flurouracil to potentiate its action
101
What are the inhibitors of topoisomerases?
Type I - Camptothecin and topotecan
| Type II - etoposides, antracyclines, mitoxantrone and epipodophyllotoxins
102
How does the herpes virus DNA polymerase function?
DNA chain is lengthened as the phosphate group at the 5' carbon of the sugar of one nucleotide subunit is linked to the OH group of the 3' carbon of the sugar of the next nucleotide
(releases PPi)
103
Camptothecins?
Anticancer drug
Topoisomerase type I inhibitor
104
Topotecins?
Anticancer drug
Topoisomerase type I inhibitor
105
Etoposides
Anticancer drug
Topoisomerase type II inhibitor
106
Epipodophyllotoxins
Anticancer drug
Topoisomerase type II inhibitor
107
Microtubules?
Protein polymer that is responsible for various aspects of cellular morphology and movement. Also involved in mitosis (pull the chromosomes by attaching to the spindles
Oscillate between polymerisation and depolymerisation (=equilibrium)
Target for anticancer drugs
108
Vinblastine
Vinca alkaloid
Anti cancer drug that binds to tubulin dimer, leading to disassembly of microtubules
This disrupts cell division
109
Taxol (paclitaxel)
Anticancer drug that bind microtubule, and thereby stabilise these structure
This disrupts cell division
110
How can steroid hormones be used against certain cancers? What kinds of cancer can this be effective for?
Hormone therapies aim to reduce the synthesis of steroid hormones and/or to anatagonise their role in signalling
Some breast cancers grow more rapidly in the presence of steroid hormone
Prostate cancers are stimulated by male steroid hormones
111
Tamoxifen
Competitive antagonist of the oestrogen receptor in breast tissue
Effective against 70% of breast cancers and inhibits cell proliferation
BUT does cause an increase in endometrial proliferation (ER agonist)
112
Toremifene
Competitive antagonist for oestrogen receptor
Used as an anti breast cancer drug
Improvement of Tamoxifen as it does not stimulate endometiral growth (ER agonism)
113
Anastrazole
Aromatase enzyme inhibitor involved in the final step of the oestrogen synthesis pathway
Inhibits oestrogen synthesis in fat and muscle of post menopausal women
Used in the treatment of breast cancer
114
Flutamide
Anti androgen drug that competes with testosterone for binding to the androgen receptor and inhibits cell proliferation
Used in the treatment of prostate cancer
115
Goseraline
Synthetic homologue of gonadotropin-releasing hormone (GnRH) that acts as a strong agonist on the GnRH receptor
GnRH indirectly stimulates testosterone synthesis in testis (via LH), but sustained agonist binding at the GnRH receptor results in complete inhibition of testosterone synthesis
116
Prednisone
Prodrug that is converted in the liver into prednisolone
| -glucocorticoid agonist used in the treatment of acute lymphoblastic leukemia
117
How can cancer cells have self-sufficiency in growth signals?
Overproduction of growth factors and or their receptors
118
Trastuzumab
Binds to the ectracellular domain of human epidermal growth factor receptor 2
(this is over expressed in 25-30% of breast cancers)
119
Cetuximab
Binds to extracellular domain of the human epidermal growth factor 1 receptor (EGFR) and prevents ligand binding
Used against colorectal cancer and head and neck cancer
120
Bevacizumab
Inhibits human vascular endothelial growth factor (VEGF). This is require of tumours to create its own blood supply
Used in the treatment of metastatic colorectal cancer (with 5-fluorouracil), lung cancer and breast cancer
121
Explain how Chronic Myeloid Leukemia comes to be
Translocation of ABL from chromosome 9 onto the Philadelphia chromosome next to BCR
Fusion protein of Bcr-Abl is expressed that contains a permanently activate tyrosine kinase (ABL1=oncogene)
122
Imatinib
Small molecule inhibitor of tyrosine kinase
Also inhibits c-kit, a receptor tyrosine kinase which is over expressed in GI stromal tumours
123
Erlotinib
Inhibits EGFR tyrosine kinase
Effective against lung cancers
124
Imantib mesylate
Agent that specifically inhibits the BCR-ABL protein kinase activity, and it is active in chronic and blast phases of CML
Can also inhibit the tyrosine kinase activity of c-kit, which is commonly over-expressed in patients with GI stromal tumours
125
What are the two forms of resistance?
Intrinsic - Inherent insensitivity to a drug
Acquired - Organisms that were originally sensitive become less sensitive or insensitive to a drug
126
How can drug resistance be created experimentally?
By growing cells in sub-toxic drug concentrations then transferring the surviving cells to a slightly higher drug concentration and allow the cells to adapt again
127
What the biochemical mechanisms of bacterial drug resistance?
```
Enzyme inactivation (inside or outside)
Target modification
Enhanced DNA repair
Metabolic bypass/target replacement
Drug efflux/secretion
```
128
How is resistance to antibiotics with a β-lactam ring achieved?
β-lactamases - hydrolyse 1000 penicilin molecules per second
129
What are good examples of intrinsic resistance?
Failure of fungi to respond to rifampin (as drug cannot pass through fungal cell envelope)
Bacteria are resistant to ergosterol as they do not synthesise ergosterol
130
Isoniazid
Inhibits the biosynthesis of mycolic acid
First line treatment against M.tuberculosis
131
How do β-lactamases act to produce drug resistance?
Their active site allows free access of water and deacetylation is fast
132
What are the classes of β-lactamases? Which one is different and why?
A, B, C and D
B is a zinc β lactamase
133
What is the difference between zinc β-lactamases and other β-lactamases?
zinc does not involve an acyl intermediate
134
How do aminoglycosides interact with their target? Why is this important in resistance?
Through the interaction of the OH and NH2 groups of the Aminoglycoside with the 16S rRNA in the 30S subunit via hydrogen bonds
The OH and NH2 groups are modified thus preventing this interaction
135
How is resistance to aminoglycosides achieved?
N-acetylation of NH2 groups by acetyl-CoA
O-phosphoryl transfer of the γ-phosphate group of ATP to an OH moiety on the aminoglycoside
O-adenylyl transfer of the α phosphate group of ATP = addition of the AMP moiety to the OH moiety on the aminoglycoside
136
How can pathogens alter the drug target to achieve drug resistance?
By mutation at one or more sites of the target gene
By enhancing expression of the target
By importation of a gene that specifies a new replacement enzyme with decreased sensitivity to drug
137
Why was methicillin developed?
It bulky side chain group enhanced the lifetime of the covalent pennicillyol-O-lactamase acyl enzyme intermediate against hydrolysis
138
What is MRSA resistant to?
Essentially all β-lactam molecules, including penicillins, cephalosporins, carbapenams, and penems
139
How does MRSA achieve its resistance?
Through the expression of the mecA gene which encodes a new β-lactam-insensitive bifunctional transglycosylase/transpeptidase
In addition to it express the fem genes which confer a high level of β-lactam resistance when expressed in combo with mecA
140
What is the function of the fem gene? What does fem stand for?
Its products add a pentaglycyl cross bridge with mixed glycine, alanine and serine residues
Produces resistance when expressed in combination of mecA
fem= factor essential for expression of methicillin resistance
141
What is the product of the mec gene?
A β-lactam-insensitive, bifunctional transglycosylase/transpeptidase
142
Which bacteria other than MRSA is showing resistance to multiple β-lactam drugs?
Streptococcus pneumonia
143
How does streptococcus pneumonia achieve β-lactam resistance?
Through changes in the composition of PG transpeptidase and other penicillin-binding proteins
144
Which bacterium is displaying effective resistance to vancomycin?
Enterococcus faecalis
145
What are the clinical phenotypes of vancomycin resistant enterococcus faecalis?
VanA and VanB
146
What is the difference between the two vancomycin resistant enterococcus faecalis phenotypes?
VanB has continuing sensitivity to glycopeptide teicoplanin (a vancomycin analogue)
147
What is necessary and sufficient to produce the Vana/VanB phenotypes?
Five tandemly arranged genes:
Three enzymes involved in the reprogramming of the PG termini to D-acyl-D-Ala-D-lactate = VanH, VanA and VanX
Two proteins, VanS and VanR forming a two component signal transducer for inducible reprogramming to vancomycin resistance
148
Resistance to which drug has been found in pneumococci?
Macrolides eg erythromycin, azithromycin and clarithromycin
149
How is resistance to macrolides achieved? What else does this cause?
Methylation of A2058 in the 23S rRNA in the 50S ribosomal subunit by RNA methyltransferase
Also reduces affinity of the rRNA for lincosamides and streptogramin B, without affecting rRNA function
150
In what species is reducing drug influx particularly effective and why?
Pseudomonsa aeruginosa because it takes in aminoglycosides by rapid facilitated diffusion through porin channels which are present in the outer membrane
151
Why does pseudomonas achieve its resistance to aminoglycosides?
Reducing the number of porins that take up the drug by facilitated diffusion and by modifying the lipopolysaccharide outer leaflet which further reduces passive influx into the periplasm
Mutations in the uptake transporters that reduce the affinity for aminoglycosides or disable it can prevent its uptake from the periplasm into the cytoplasm
152
What drug is often given in combination with β-lactam drugs in the treatment of S. aureus? Why?
Aminoglycosides
Because they can passively diffuse into the cell more easily due to the inhibition of cell wall synthesis
Helps to prevent the development of resistance by increasing the bactericidal action
153
How does increasing the hydrophobicity of a drug help to prevent resistance?
Drug influx cannot be prevented due to the non-protein mediated diffusion
154
How are hydrophobic drugs removed from cells?
By active drug efflux
155
For which drugs is active drug efflux clinically relevant?
Β-lactams, macrolides, fluroquinolones, tetracyclines and many more
156
How can the specificity of drug efflux transporters vary?
Can be specific to a single drug/ class of drug or can exhibit an extremely broad drug specificity
157
What is an example of a transporter that is effective for a single drug?
The tetracycline transporter
158
What are the two main ways by which bacteria can get a remarkable capacity to efflux drugs?
Chromosomally encoded metabolic capacity
Or by the acquisition of transport genes on plasmids and transposons
159
What are the two kinds of drug transporter?
Primary - ATP hydrolysis
Secondary - Mediate the outward movement of drugs to the inward movement of ions (H+/Na+)
160
Generally speaking within which organisms use the two kinds of drug transporter?
Primary is mainly used by eukaryotes
Secondary is more commonly used by prokaryotes
161
What is the general structure of the drug transporters found in Gram negative bacteria?
Secondary active transporter often associated with an accessory protein which spans the periplasm, and an outer membrane porin to allow drug transport across the cell envelope
162
What is multiple drug resistance?
The simultaneous expression of various antibiotic resistance mechanisms, each specific for a drug or class of drugs
They genes can be activated by regulon
163
What is multidrug resistance?
The presence of a multidrug efflux pump in the plasma membrane confers resistance to a wide variety drugs due to the enormously broad specificity of the pump
164
How can resistance to azoles arise?
Alterations in the activity and amount of enzymes involved in ergosterol biosynthesis
Active azole efflux
165
How is resistance to purine analogues achieved?
Changes in the substrate specificity of the viral purine-activating thymidine kinase, disabling the phosphorylation of purine analogues.
166
How is resistance to reverse transcriptase inhibitors or protease inhibitors achieved?
Due to mutations in these enzymes that disable the interaction between enzyme and inhibitor
167
How is chloroquine resistance achieved?
Through less efficient accumulation of chloroquine in their food vacuoles = exclusion of the drug from the site of action
Found to be the result of pumping the drug out - either by an ATP-dependent P-glycoprotein or CRT (chloroquine resistance transporter)
168
How can drug resistance be achieved in cancer?
1) Drug detoxification by cytochrome p450 systems (CyP450) and on conjugation by glutathione S-transferase and other conjugating systems
2) Mutations in drug targets
3) Increases in DNA repair pathways
4) Metabolic bypass
5) Drug efflux by multidrug transporters
169
Give an example of how cancers can develop resistance to drugs via mutations in drug targets
Cells resistant to topo poisons (eg etoposides) possess modified topoisomerases
170
Give examples of how cancers can develop resistance to drugs via an increases in DNA repair pathways.
Nitrosurea resistant cells have high levels of alkyltransferases that repair guanine lesions and so prevent DNA cross-linking
Cisplatin resitnat cells have higher levels of enzymes involved in DNA repair
171
Give an example of how cancers can develop resistance to drugs through metabolic bypass
Methotrexate reistance can be based on enhanced expression of dihydrofolate reductase
(can also result from reduced uptake due to mutations in the folate carrier which reduces the affinity for methotrexate)
172
Give examples of some drug efflux pumps used by cancer cells
Multidrug resistance P-glycoprotein MDR1
Multidrug resistance associated proteins (MRP1, MRP2)
Breast cancer resistance protein (ABCG2)
173
What are the strategies used to combat drug resistance?
Identification of new drug targets
Specific inhibitors of drug resistance mechanisms
Combination therapies
Extending antibiotic lifespan
174
What is the current mechanism used for identifying new drug targets
Sequencing the genome and looking for essential for survival/virulence
Then tests their protein products against inhibitors
Potential inhibitors can then be tested in vitro and vivo
175
Give some examples of specific inhibitors of drug resistance mechanisms
Clavunate = suicide substrate for β lactamase (used in augmentin)
Sulbactam forms covalent acyl intermediate with β lactamase (used in Unasyn)
176
Augmentin
Combination of amoxicillin (β lactamase) and clavulanate (β lactamase inhibitor)
177
Ly333328
Semisynthetic analogue of vancomycin
Contains a hydrophobic biphenyl substituent on the vancosamine sugar and is more hydrophobic and pay partition the analogue more to the membrane, as well as alter its ratio of inhibition between transpeptidases and transglycosylase
178
Oxazolidinones
New class of antibiotic with broad spectrum
Also inhibit protein biosynthesis, specifically by interaction with the 23S ribosomal RNA of the 50S subunit (at or near the peptidyl transferase centre)
179
Linezolid
Oxazolidinone
Interacts with the 23S ribosomal subunit of the 50S subunit at or near the peptidyl transferase centre of the ribosome
180
Why is combination therapy so effective?
Because it reduces the chance of them developing resistance
181
What are the attempts to extend anitbiotic lifespans (preventing resistance)
Judicious use of antibiotics
| Rotating the use of antibiotic
182
How can PARP inhibitors be used as anti-cancer drugs?
PARP = single strand repair
Therefore is BRCA2 is not functioning then blockage of PARP = cell death
183
BMN-673
PARP inhibitor in Phase III clinical trials for BRCA2 mutated breast cancer
185
Olaparib
A PARP inhibitor in phase III clinical trials
186
Isoniazid
First-line medication used to treat TB
Inhibits the biosynthesis of mycolic acid in Mycobacterium tuberculosis
187
Bacitracin
Cyclic polypeptide antibiotic
Forms a tight complex with Mg2: and bactoprenol phosphate
Inhibits the dephosphorylation to bactoprenol phosphate
188
Clavulanate
A β lactamase inhibitor used in combination with amoxicilin
189
Aminocoumarins
Class of antibiotics that act by inhibiting type II DNA topoisomerases
190
Nystatin
Antibiotic/antigunal agent
Binds to the sterol ergosterol and facilitates the formation of pores
191
Fluconazole
Triazole antigunal drug
Inhibits ergosterol biosynthesis
192
Miconazole
Imidazole antifungal and antiprotozoal drug
Inhibits ergosterol biosynthesis
193
HAART
Combination of nevirapine, zidovudine and saquinavir used to treat serious manifestations of HIV infections in patients with AIDS
